The disclosures of the following priority application is herein incorporated by reference:
Japanese Patent Application No. 2001-319813 filed Oct. 17, 2001.
1. Field of the Invention
The present invention relates to a super wide-angle lens system having a large angle of view and, in particular, to a large aperture, retrofocus type, super wide-angle lens system having a floating adjustment mechanism and a shooting device using the same.
2. Related Background Art
In the large aperture, super wide-angle lens system proposed so far, it has been scarce that has a large aperture of f/1.4 and a covering angle (an angle of view) of 2xcfx89=94xc2x0 or more.
As a focusing method for a large aperture, super wide-angle lens system, Japanese Patent Application Laid-Open No. 1-134411 discloses an internal focusing system for a large aperture, super wide-angle lens system, in which the focusing is carried out by moving a lens group in the lens system.
Moreover, the inventor of the present application disclosed a rear focusing, large aperture wide-angle lens system in Japanese Patent Application Laid-Open No. 7-35974. In the system, focusing is carried by moving only the most image side lens group in the lens system.
As described above, a lens system having a covering angle (an angle of view) of 2xcfx89=94xc2x0 or more and an f-number of 1.4 has been scarcely proposed in patent applications. A lens system equipped with those specifications does not exist on the market.
The present invention proposes a large aperture, super wide-angle lens system having a covering angle (an angle of view) of 2xcfx89=94xc2x0 or more and an f-number of 1.4, which has been never yet proposed. In order to develop a large aperture, super wide-angle lens system, there are technical problems such as compactness for general use, high optical performance, securing a peripheral quantity of light, using an aspherical surface having high productivity, and, in particular, controlling variation in aberrations upon focusing at a close or short distant object relative to aberrations upon focusing at an object at infinity, of a super wide angle lens system having extremely large angle of view. This variation is referred to as variation in close-range aberration or close-range aberration variation hereinafter.
The larger the aperture is, the smaller the maximum permissible aberrations for high optical performance are. Difficulty in correcting close-range aberration variation in addition to difficulty in improving optical performance upon focusing at infinity makes it further difficult to accomplish a lens system having the specifications. Accordingly, using only conventional technique cannot realize satisfactory result.
From this point of view, the lens system disclosed in Japanese Patent Application Laid-Open No. 1-134411 has small angle of view and is rather large in comparison with other conventional lens system. In addition, if you try to make the lens system to have a wider angle of view, the lens system becomes further larger. Moreover, correction of astigmatism and coma is not sufficient, so variation in aberrations upon focusing is not small sufficiently. Furthermore, since the lens system carries out focusing by moving inner lens group of a retrofocus type lens system, the lens system unavoidably becomes large in order to secure dead space.
On the other hand, no matter how expands the lens designing technique, a lens design having high optical performance and compactness cannot be derived from the lens system disclosed in Japanese Patent Application Laid-Open No. 7-35974.
The present invention is made in view of the aforementioned problems and has an object to provide a large aperture super wide-angle lens system having an angle of view of 2xcfx89=94xc2x0 or more, an f-number of about 1.4, small variation in close-range aberrations, compactness, and high optical performance.
According to one aspect of the present invention, a super wide-angle lens system includes, in order from an object, a negative lens group GN having negative refractive power which is fixed upon focusing, and a positive lens group GP having positive refractive power as a whole which is moved upon focusing. The positive lens group GP includes a front lens group GF having an aperture stop and a rear lens group GR having positive refractive power. Focusing at a short distant or close object is carried out by moving the front lens group GF and the rear lens group GR to the object side with different moving amounts. The following conditional expression (1) is satisfied:
0.1 less than XR/XF less than 1.0xe2x80x83xe2x80x83(1) 
where XF and XR denote the moving amounts of the front lens group GF and the rear lens group GR, respectively, when focusing at an any close object.
In one preferred embodiment of the invention, the following conditional expression (2) is satisfied:
0.1 less than |FR/FF| less than 5.2xe2x80x83xe2x80x83(2) 
where FF denotes the focal length of the front lens group GF, and FR denotes the focal length of the rear lens group GR.
In one preferred embodiment of the invention, the following conditional expression (3) is satisfied:
1.0 less than xe2x88x92FN/FP less than 5.0xe2x80x83xe2x80x83(3) 
where FN denotes the focal length of the negative lens group GN, and FP denotes the focal length of the positive lens group GP.
In one preferred embodiment of the invention, the negative lens group GN includes at least one negative lens element and at least one positive lens element locating to the image side of the negative lens element. The negative lens element has an aspherical surface facing to a concave surface side. When the aspherical surface is expressed by the following expression:                               X          ⁡                      (            y            )                          =                  xe2x80x83                ⁢                                            (                                                y                  2                                /                r0                            )                        /                          [                              1                +                                                      (                                          1                      -                                              κ                        ·                                                                              y                            2                                                    /                                                      r0                            2                                                                                                                )                                                        1                    /                    2                                                              ]                                +                                                  xe2x80x83                ⁢                              C3            ·                          "LeftBracketingBar"                              y                3                            "RightBracketingBar"                                +                      C4            ·                          y              4                                +                      C6            ·                          y              6                                +                      C8            ·                          y              8                                +                                                  xe2x80x83                ⁢                                            C10              ·                              y                10                                      +                          C12              ·                              y                12                                      +                          C14              ·                              y                14                                              ,                    
the following conditional expression (4) is satisfied:
xe2x88x921.0 less than xcexa less than 1.0xe2x80x83xe2x80x83(4) 
where X(y) denotes the displacement (sag amount) in the optical axis direction from the tangent plane of the vertex of the aspherical surface at the height y from the optical axis in a vertical direction relative to the optical axis to the aspherical surface, r0 denotes the reference radius of curvature, K denotes the conical coefficient, Cn denotes an n-th order aspherical coefficient.
In one preferred embodiment of the invention, the front lens group GF includes a negative lens element located to the object side of the aperture stop. The negative lens element has an aspherical surface including a 3rd order aspherical coefficient C3. When the aspherical surface is expressed by the following expression:                               X          ⁡                      (            y            )                          =                  xe2x80x83                ⁢                                            (                                                y                  2                                /                r0                            )                        /                          [                              1                +                                                      (                                          1                      -                                              κ                        ·                                                                              y                            2                                                    /                                                      r0                            2                                                                                                                )                                                        1                    /                    2                                                              ]                                +                                                  xe2x80x83                ⁢                              C3            ·                          "LeftBracketingBar"                              y                3                            "RightBracketingBar"                                +                      C4            ·                          y              4                                +                      C5            ·                          "LeftBracketingBar"                              y                5                            "RightBracketingBar"                                +                                                  xe2x80x83                ⁢                              C6            ·                          y              6                                +                      C7            ·                          "LeftBracketingBar"                              y                7                            "RightBracketingBar"                                +                      C8            ·                          y              8                                +                                                  xe2x80x83                ⁢                                            C10              ·                              y                10                                      +                          C12              ·                              y                12                                              ,                    
the following conditional expression (5) is satisfied:
1.0xc3x9710xe2x88x926 less than |C3| less than 1.0xc3x9710xe2x88x922xe2x80x83xe2x80x83(5). 
In one preferred embodiment of the invention, the negative lens element locating the most object side of the negative lens group GN having an aspherical surface which is a hybrid aspherical lens constructed by a glass material and a resin material. The following conditional expression (6) is satisfied:
0.0015 less than (dmaxxe2x88x92d0)/F0 less than 0.5xe2x80x83xe2x80x83(6) 
where d0 denotes the thickness along the optical axis of the resin material forming the aspherical surface, dmax denotes the thickness of the resin material in the direction of the optical axis where the thickness becomes maximum within the effective aperture, and F0 denotes the focal length of the super wide-angle lens system.
According to another feature of the present invention, a shooting device uses the super wide-angle lens system.